Plants ax Affected by Insufficient Food. 149 



constituent of the air, plays no direct part in plant nu- 

 trition (260). 



256. The Sources of Nitrates in the Soil are 



a Nitrification (nit-ri-fi-ca'-tion), by which the nitro- 

 gen contained by organic matter and ammonium sulfate 

 in the soil is changed to nitric acid through the agency 

 of microscopic plants (bacteria). The nitric acid thus 

 formed combines with certain substances (bases') in the 

 soil, as potash and lime, forming nitrates (255). 



b Symbiosis (sym-bi-o'-sis) on the roots of legumin- 

 ous crops, through which atmospheric nitrogen is changed 

 to nitric acid (260, 113). 



c Deposits from the atmosphere in rain or snow (261). 



d Ammonium salts or nitrates applied to the soil (262). 



257. The Conditions Affecting Nitrification are similar 

 to those affecting plant life in general, since nitrification 

 results from plant life. As it takes place below the sur- 

 face of the soil, it is favored by the same conditions that 

 favor the root growth of land plants, viz., aeration, 

 warmth and moisture. In general, it is active during 

 the growing season, but at a standstill during the dor- 

 mant period. It does not proceed rapidly in spring un- 

 til the soil has become sufficiently warm to promote 

 active root growth. 



Nitrification also releases the other food materials con- 

 tained by organic matter (93). 



258. Soil Aeration Promotes Fertility by favoring nitri- 

 fication. Thus cultivation and drainage (of heavy soils) 

 not only directly promote the growth of plants by assist- 

 ing aeration (94), but they actually increase plant food. 

 Early plowing in spring promotes nitrification by favor- 



